In recent years, where protection of the global environment has been sought, reduction of exhaust gas from automobiles, a major factor in air pollution and global warming, in particular reduction of the amount of exhaust of carbon dioxide per unit distance traveled, has become an urgent task. In order to reduce the amount of exhaust of carbon dioxide, fuel consumption has to be lowered. To lower fuel consumption, reduction of the weight of vehicles is extremely effective.
For forged parts and machined parts of ferrous materials used for the engine and chassis among auto parts, in the past, carbon steel, alloy steel, and V-containing microalloyed steel have been used. These steels have compositions of 97% or more of Fe and elements such as Mn, Cr, and V having specific gravities equal to or greater than that of Fe, and therefor these steels have specific gravities of around 7.8.
Auto parts had been reduced in weight by strengthening the steel and thereby enabling increased thinness or changes in part shapes based on the assumption of a constant specific gravity of the materials. However, in recent years, reduction of the specific gravity of steel has been studied. Several proposals have been made regarding low specific gravity steel mainly comprised of Fe.
As examples of low specific gravity steel mainly comprised of Fe, for example, there are the automobile-use steel sheets containing large amounts of Al described in PLTs 1 and 2. PLT 1 describes high strength, low specific gravity steel sheet containing C: over 0.01 to 5%, Si: 3.0% or less, Mn: 0.01 to 30.0&, P: 0.1% or less, S: 0.01% or less, Al: 3.0 to 10.0%, and N: 0.001 to 0.05% and having a specific gravity of <7.20 and a value TS×El of a product of tensile strength TS (MPa) and elongation at break El (%) of 10000 MPa·% or more. Further, PLT 2 discloses high strength, low specific gravity steel sheet having a similar composition to the steel sheet of PLT 1, having Al of over 10 to 32.0%, and, furthermore, having a low specific gravity.
The steel sheets of the PLTs 1 and 2 are produced by treating Al-containing steel which contains a trace of P and S reduced in elements which make grain boundary embrittle, are produced through structure refinement process such as recrystallization by setting final rolling temperature at 950 to 960° C., and adjusting the coiling temperature to improve the workability of the steel sheets. As a result, the steel sheets have sufficient ductility. In this way, in a steel sheet produced by hot rolling, the structure can be made finer by controlling the rolling conditions in the rolling process, so it is possible to produce steel containing a relatively large amount of Al as a raw material.
On the other hand, the general process of hot forging comprises only heating a steel bar to a temperature of about 1200° C. or more, then forging it finishing at about 1100° C., then cooling it in accordance with the properties of the steel material. So, when the steel containing a large amount of Al is hot forged, such a structural control done with steel sheet is not possible in forging process, so the structure after forging becomes coarse and the strength and toughness become inferior.
Rolled steel sheet and hot forged products have the above such differences, so not all of the steels described in PLTs 1 and 2 can be applied as materials for hot forging use. Furthermore, even if the steel can be hot forged, the machinability, which is necessary for steel for forging use, is not sufficient.
For example, in forged parts such as automobile chassis parts, a high tensile strength of 800 MPa or more is demanded and, at the same time, superior machinability enabling mass production is required, in many cases. In the steels described in PLTs 1 and 2, the machinability is not considered at all. In particular, in the case assuming machining, the amount of S is completely insufficient.
Furthermore, as another example, there is the iron alloy described in PLT 3. PLT 3 describes a low specific gravity iron alloy comprised of Mn: 5.0 to less than 15.0%, Al: 0.5 to 10.0%, Si: 0.5 to 10.0%, and C: 0.01 to 1.5% and provided with a γ+α two-phase structure having an α phase fraction of 10 to 95%.
In this iron alloy, Al is increased to reduce the specific gravity and furthermore mainly the Mn is raised to stabilize the γ phase and finally form a γ+α two-phase structure having 10 to 95% of an α phase. By this, a high specific strength and workability are obtained. In particular, a superior cold workability is obtained with an α fraction of about 60% or less. The hardness and cold workability of this iron alloy are largely dependent on the ratio of γ and α. For industrial use, it is necessary to stably adjust the ratio of γ and α. However, it is extremely difficult to precisely obtain the targeted γ/α ratio hot working and various heat treatment processes. There is therefore the problem that iron alloy and the production process described in PLT 3 is not suited to industrial production. Furthermore, this alloy has as its object to obtain a superior hardness. It does not contain S and does not consider machinability at all.
Above, Al-containing steels for various structural uses were explained. Viewing Al-containing steels as a whole, the main applications utilize their corrosion resistance, high temperature oxidation resistance, and vibration damping properties. As one example, PLT 4 may be mentioned. PLT 4 discloses an Fe—Mn—Al alloy as an inexpensive alternative steel to stainless steel.